Background Art
Aircraft landings in fog, rain, haze and other inclement weather conditions tending to obscure a pilot's view of a runway during a landing approach are controlled by FAA regulations for commercial aircraft and by military regulations at military airfields. In the absence of an appropriate all-weather instrument landing system (ILS), landing restrictions are based on the distance at which the runway may be visually discerned by the pilot of the aircraft. This distance is called the "runway visible range" (RVR) when no landing aid is employed. There are two major effects which limit the RVR: the extinction coefficient of the intervening fog, clouds or haze; and the masking radiation scattered to the observer from sources other than runway lights. The masking radiation includes backscatter from the sun, moon, aircraft lights and scatter from radiation sources on the ground. Backscatter from the sun, moon and aircraft head lamps is mostly time invariant over periods of tenths of a second, with some approximately random fluctuations. Aircraft wing and tail lamps are periodic with periods long compared to about 25 Hz. Backscatter from sources on the ground is usually from either arc lamps, incandescent lamps, or fluorescent lamps. These have a DC component, and one at twice the power line frequency (2f.sub.p) plus harmonics. Examples of approximate extinction coefficients for various RVR's are given in the following table:
______________________________________ Runway 500 ft. 700 ft. 1100 ft. 2100 visual range 152.5 m 213.5 m 335.5 m 640.5 m ______________________________________ Extinction 0.03246 0.02040 0.01091 0.00403 coefficient, day (m.sup.-1) Extinction 0.08721 0.05883 0.03477 0.01619 coefficient, night (m.sup.-1) ______________________________________
The RVR for a given landing category also varies somewhat from airport to airport. The following approximate values are typical:
______________________________________ Category Minimum RVR ______________________________________ Cat I 1800-2400 ft. (549-732 m) Cat II 1200 ft. (366 m) Cat IIIa 700 ft. (213.5 m) Cat IIIb 300 ft. (91.5 m) Cat IIIc 0 ft. ______________________________________
When the RVR is less than a minimum distance set by the FAA for commercial aircraft, or by the military for military aircraft landing at military airfields, the aircraft will not be allowed to land. Obviously, this can cause significant delays. With a commercial aircraft, the aircraft may need to be rerouted to an airport in another city where weather conditions permit landing the aircraft. In military applications, military aircraft such as military transport aircraft must be able to land near a battlefield and often at airfields with limited support systems. Frequently, either the aircraft or the airfield, or both, are not equipped with the appropriate all-weather instrument landing systems needed to safely land an aircraft under obscured visual conditions. Since all- weather instrument landing systems are also expensive to install, there exists a need for an alternative system and method for enabling a pilot of an aircraft, whether military or commercial, to adequately visualize a runway during poor weather conditions in order to land the aircraft.
While various apparatus have been developed in an attempt to aid a pilot in visualizing a runway during weather conditions obscuring the pilot's vision, such systems have generally proven to be fairly expensive and/or complicated to install on the aircraft or at an airfield. Examples of various attempts at implementing systems for aiding pilots in landing aircraft during conditions of reduced visibility at an airfield are disclosed in the following patents, the disclosure of each of which is hereby incorporated by reference:
______________________________________ 1,936,400 4,210,930 3,510,834 4,419,731 3,643,213 4,866,626 3,671,963 4,868,567 3,952,309 5,559,510 ______________________________________
In view of the above, it would be highly desirable to provide a system which increases the distance at which a runway is visually discernible during weather conditions such as fog, rain and haze, which would otherwise reduce the RVR to a distance which would prevent landing the aircraft.
It would further be desirable to provide a system which is relatively inexpensive and which can be installed relatively quickly at an airfield and on an aircraft, and without major modification to the airfield or aircraft, to aid a pilot in viewing the runway during weather conditions which obscure the pilot's view of the runway, to thereby enable the aircraft to be landed during weather conditions which would otherwise reduce the RVR to a distance preventing the aircraft from being landed at the airfield. It would also be desirable if such a system could be employed without the need for the aircraft to transmit signals, such as electromagnetic signals, which in military applications could make the aircraft electronically detectable by an enemy.
It would further be desirable to provide a system which enables the various runways and taxi areas of an airport or airfield to be illuminated in such a manner as to make each distinguishable from the others, and a means provided for enabling a pilot of an aircraft to discern between one or more runways or taxi areas in conditions of limited visibility.